We propose to continue the study of mechanisms of repair of UV-damaged DNA in two bacterial species, Escherichia coli and Bacillus subtilis. In particular, we plan to extend our studies of post-replication repair in excision-deficient cells, using a dimer-specific endonuclease to determine the fate of dimers during repair. We are expecially interested in the origin and significance of high molecular weight daughter DNA apparently copied from irradiated template molecules which still contain UV-induced dimers. We wish to determine whether the production of this DNA reflects a rapid filling of gaps in discontinuous daughter strands synthesized after irradiation, or some other repair mechanism, perhaps a dimer by-pass. BIBLIOGRPAHIC REFERENCES: Ganesan, A.K. and Seawell, P.C. (1975) The effect of lexA and recF mutations on post-replication repair and DNA synthesis in Escherichia coli K-12. Molecular Gen. Genetics 141: 189-205. Seawell, P.C., Friedberg, E.C. and Ganesan, A.K. (1977) Further Characterization of T4 endonuclease V by electrophoretic analysis of superhelical DNA on agarose gels. Biophysical Society Abstracts.